This invention relates to optical devices and methods and, more particularly, to the spectral analysis of images.
The features in a scene may often be analyzed most effectively according to the optical spectra that they produce at various wavelengths, as well as their physical size, shape, and position. Most features have characteristic optical spectra that may be used to distinguish them from other features. The optical spectra vary according to wavelength, so that an array of optical spectra over a range of wavelengths provides a unique signature pattern for the feature of interest.
The human eye integrates these spectra so that it perceives only an averaged color pattern for the feature. The human eye may therefore not be able to spectrally distinguish many types of features, such as a target feature that is painted green from the green of a grassy field in which the object is located. The result is that the target feature is effectively invisible to the naked eye against the background.
On the other hand, a spectrometer produces a spectral pattern of magnitude of the incident optical signal as a function of its wavelength. An imaging spectrometer performs this same function at each location over the field of view of a scene. The optical spectra may be analyzed to distinguish a target feature of interest from the background and, if necessary, to compare the optical spectra of the target feature with known spectral forms to identify the nature of the target feature. For example, a green-painted target feature may be located in a green background because it has a different optical spectrum than does the background. The nature of the target feature may be identified by comparing its target optical spectrum with those in a library of optical spectra to determine whether the target feature is a person clothed in green, a green tree, a green wooden structure, a green metal object, etc. Military seekers may make use of these capabilities to locate and identify camouflaged target features.
One of the problems encountered in the use of imaging spectrometers is that they collect too much data to be processed effectively in real time. For example, an imaging spectrometer with a focal plane array of 500xc3x97500 pixels collecting 100 spectral bands 60 times per second with 14 bits of digital resolution has a data rate of 21 gigabits per second. The extraction of useful information typically requires 10 to 100 operations per pixel, for a required operational rate of the data processor of 15-150 giga-operations per second. The computing power now available and which may be expected to be available in the near future cannot support these data processing rates.
To overcome this problem, prior imaging spectrometers have taken several approaches for limiting the data taken. In some cases, only a few selected spectral bands are collected, so that the amount of data to be analyzed is manageable. However, this approach is not fully satisfactory because the portions of the optical spectrum required to distinguish target features from the background may vary widely. For example, distinguishing target features against an earth background and against a water or sky background require the analysis of different portions of the optical spectrum. The limiting of the spectral analysis to a few bands prevents the locating and identification of features over a wide range of conditions. Other approaches involve limiting the spatial extent of the imaged scene or increasing the time taken to collect the data, but these approaches result in lower resolution, lower field of view, and/or slower speed.
There is a need for an improved approach to the spectral analysis of images in order to maintain data analysis rates within the capabilities of available and expected data processors, and also to provide robust analysis capabilities operable over a wide range of conditions. The present invention fulfills this need, and further provides related advantages.
The present invention provides an adaptive spectral imaging device and a method for adaptively analyzing a scene. This approach permits the analysis of a scene over a full spectral range to locate and identify target features in the scene, while holding the amount of data processing within the capabilities of existing and expected data processors.
In accordance with the invention, an adaptive spectral imaging device operable to analyze a scene comprises a detector such as a focal plane array having a detector output signal, and an optical system disposed in an optical ray path between the scene and the detector. The optical system images the scene onto the detector. A controllable optical disperser is disposed along the optical ray path between the scene and the detector. That is, the optical dispersion of the optical disperser is controllable. The controllable optical disperser is preferably an optical phased array, but could be other types of controllable devices as well. In a typical configuration, the optical system contains a first lens and a second lens, and the controllable optical disperser is positioned between the first lens and the second lens. The controllable optical disperser has a disperser input command signal. A controller has a controller output signal responsive to the detector output signal. The controller output signal is provided to the controllable optical disperser as the disperser input command signal.
In one application, the controller includes a calculational routine that analyzes low-resolution (spatial and spectral) scene information and uses that information to modify (via the adaptive disperser) the spectral and optionally the spatial content of the detected imagery. The modification discards spectral and spatial information that is not relevant to the task being accomplished. This winnowing of information results in much less data to process and thus allows the decisions based upon the spectral content of the scene to be made much more rapidly. An example task of interest is to produce an image that has a high contrast between a camouflaged object and a cluttered natural background. The object might be a tank in the trees or an intruder in the bushes. An example calculational routine is a whitening operation conducted using multiresolution techniques.
Thus, a method for adaptively analyzing a scene comprises the steps of imaging a scene onto a detector so that a ray path from the scene to the detector passes through a controllable optical disperser. The controllable optical disperser is controlled responsive to an output signal of the detector.
Existing imaging spectrometers use a fixed optical disperser, such as a prism or a diffraction grating, whose dispersion characteristics are not controllable. The data analysis attempts to utilize all of the data produced, with the result that there is too much data to be analyzed in real time using available computational capabilities. In the present approach, on the other hand, the optical disperser is controllable so that only the most useful spectral information is gathered and need be processed. The spectral information that is xe2x80x9cmost usefulxe2x80x9d depends upon the circumstances. The controller makes that determination based upon the information received from the detector, and commands the operation of the controllable optical disperser as needed to produce the most useful information.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.